1. Field of the Invention
The present invention relates to a process for producing an amorphous alloy material which is improved in the prevention of embrittlement peculiar to an amorphous alloy when the amorphous alloy material is subjected to a prolonged thermal history in a high-temperature working.
2. Description of the Prior Art
The present inventors invented an Al-TM-Ln alloy and Mg-TM-Ln alloy (wherein TM is a transition metal element or the like and Ln is a rare earth metal element or the like) as light-weight high-strength amorphous alloys and applied for a patent as disclosed in Japanese Patent Laid-Open Nos. 275732/1989 and 10041/1991, respectively. In addition, they also invented an Al-TM-Ln alloy and a Zr-TM-Al alloy as high-strength amorphous alloys having excellent workability and applied for a patent as disclosed in Japanese Patent Laid-Open Nos. 36243/1991 and 158446/1991, respectively.
Having high strength and high corrosion resistance, each of the above alloys shows glass transition behavior and possesses a supercooled liquid region, and therefore exhibits favorable workability at a temperature within or around the above region. Moreover, the above alloys that are obtained as powder or thin ribbon can be easily subjected to consolidation forming, and further made into amorphous bulk materials by casting, and are also excellent alloys exhibiting favorable workability at a temperature within or around the supercooled liquid region.
However, when the amorphous alloy is held in the above-mentioned supercooled liquid region for a long time, it begins to be transformed to its crystalline form, thus restricting the time for working such as consolidation forming and work forming. As the countermeasure against the restriction, there is available a method in which consolidation forming and work forming are carried out at the glass transition temperature or lower. As is the case with the general amorphous alloys, the aforestated alloy, when heated to a temperature immediately below the glass transition temperature, suddenly loses the ductility peculiar to the amorphous alloys and is inevitably embrittled. Accordingly, the amorphous alloy subjected to consolidation forming or rework forming at a high temperature still involves the problem of failure to sufficiently exhibit the inherent characteristics thereof.
The present inventors have found that the ductility of the alloy is restored by a two-step treatment which comprises holding the alloy in the supercooled liquid region (glass transition temperature region) after working immediately below the glass transition temperature and subsequently quenching the alloy for the purpose of solving the above-mentioned problem, and applied for a patent on the basis of the aforesaid finding. Thereafter, they have further found out a method which can dispense with the quenching after the second stage heat treatment. The present invention has been accomplished on the basis of the above finding.
It has already been known that in general an amorphous alloy is embrittled when heated to a high temperature around the glass transition temperature thereof even if the temperature is lower than the crystallization temperature thereof.
The above phenomenon is attributable to the structural change into a more stable atomic arrangement in spite of its amorphousness and generally relates to structural relaxation. In the above structural relaxation a reversible reaction and an irreversible reaction are mixed with each other, of which the reversible reaction is cancelled by rapid heating to a high temperature. However, the abovementioned phenomenon takes place in an extremely short time and successively brings about further structural relaxation at another temperature. Consequently it is impossible to prevent the structural relaxation of an amorphous alloy by reheating alone, thus making it difficult to avoid such structural relaxation.